1. Field of the Invention
The present invention relates to a process for the production of piping made of a cementitious material having a circular section.
The present invention derives from the field of extrusion processes of tubular-shaped end-products made of a cementitious material.
In particular, the present invention relates to a process for the production by extrusion of piping made of fibre-cement with a circular section and fine thickness, suitable for the channeling of liquids and gases at atmospheric operating pressure or slightly higher. The present invention also allows the production of end-products with a circular section, for applications in the building and industrial sector such as for example permanent formworks and pillars.
2. Description of Related Art
In the transportation of drinking water, irrigations and wastewater, various types of pipes or ducts are normally used, made of different kinds of materials such as: cementitious materials, plastic materials, concrete, ceramic stoneware and cast iron.
Typical pipes made of cementitious material are pipes made of concrete, reinforced concrete, asbestos cement and fibre-cement without asbestos. The most widely-used plastic materials, on the other hand, are PVC, polyethylene, polypropylene and glass-resin.
As far as the shape of the pipes is concerned, those with a circular section are the most commonly used. There are also however pipes have different shapes from circular, such as for example, circular footrest pipes (flat bottom), elliptic or ovoidal pipes, rectangular pipes, or pipes with other sections specifically designed for favouring the maximum fluid flow in their interior.
The diameters available for the pipes can vary and are divided according to the various types of use.
Another important construction characteristic of these pipes consists of their thickness; those having a so-called “fine thickness”, typically have a vacuum percentage of the section higher than 60%.
With respect to processes for the production of piping made of cementitious material, these have been known from the beginning of the last century.
In 1910, W. R. Hume described, in Australian patent 4843/2622, a process for the production of reinforced concrete pipes by means of centrifugation exploiting the centrifugal force. A cylindrical mould with a horizontal axis charged with concrete was rotated at a high velocity, with the removal of the excess water until a compact material was obtained. The so-called “Hume pipes” are still produced, still exploiting the technique based on centrifugation, optionally using reinforced concrete with steel fibres, or other compositions.
Another production technique used in the past is that called “Rotopress” or “Giropress” whereby pipes were produced in vertical, by a rotating mandrel which packed the concrete having a consistency of the humid earth type in an axial direction.
This system has now been substituted by other production technologies such as, for example, the vibrocompression technology, in which dry concrete is again used. In this case, the pipe produced in vertical is immediately removed from the mould and sent to the curing phase.
With the type of production technologies so-far cited, pipes having relatively high thicknesses are obtained, which comply with the European regulation EN 1916 (reinforced concrete, non-reinforced concrete, concrete reinforced with steel fibres).
In addition to concrete pipes, pipes made of fibre-cement having a fine thickness are also known, mainly produced by means of the so-called Mazza process (deriving from the Hatschek technology). In this case, the material used par excellence, was asbestos-cement, recently substituted for environmental reasons by so-called fibre-cement. In the Mazza/Hatschek process, cementitious compositions are used, containing cement, process fibres and reinforcing fibres (both synthetic and natural) and other secondary additives. The products obtained have high mechanical characteristics, they are extremely compact and have low thicknesses.
More recently, the use of the extrusion technology has been proposed, widely used for plastic materials, metals, ceramics, ceramic stoneware and bricks, and also for cementitious materials. The extrusion can be effected with batch or intermittent plug/cylinder systems (“plug extrusion”, or “capillary extrusion”), or with continuous screw/cylinder systems. With the exception of ceramic stoneware, in all the other cases the extrusion is carried out horizontally. In the case of ceramic stoneware, in fact, thanks to the high thicknesses of the pipes in relation to their length (normally two meters), there is a rigidity in the fresh state of the pipes which does not cause deformation or distortion.
As far as the extrusion of cementitious materials is concerned, the known art refers to extruders having two consecutive screws, intervalled by a vacuum chamber to facilitate the pressurized extrusion of pastes. These are extruder models normally used in the brick industry.
Extrudable cementitious compositions for the production of pipes made of cementitious materials are described in U.S. Pat. No. 3,857,715 issued in 1974 in the name of C. W. Humphrey, and U.S. Pat. No. 5,047,086 issued in 1991 in the name of K. Hayakawa et al.
The U.S. Pat. No. 5,658,624 of 1997 in the name of Anderson et al. describes compositions and methods for producing a variety of articles based on extrudable hydraulic cement.
U.S. Pat. No. 5,891,374 of 1999 of Shah et al., which describes the extrusion of reinforced-fibre products, is also known.
U.S. Pat. No. 6,309,570 of Fellabaum et al. describes a vacuum system for improving the extrusion of cementitious products, without referring however to tubular products.
The extrusion of a reinforced-fibre with a pseudoductile behaviour for the production of low-thickness pipes, is also known from international patent application WO 2005/050079. This international patent application makes reference to a particular extrusion technique previously described in U.S. Pat. No. 6,398,998 B1 which does not exploit the screw system for the extrusion phase, but a water suction method from a liquid reinforced-fibre cementitious formulation, introduced under pressure into a kind of coaxial cylinder. After the water extraction, the material is formed at a high pressure, obtaining pipes having a particularly fine thickness with extremely valid mechanical properties, in terms of ductility.
The US patent application 2004/0075185 A1 di Dugat et al. which relates to a plug moulding system of a high performance cementitious material for producing sewage pipes with a medium-high thickness, is also known. The technology described is also known by the name of Tetris or Evolit.
The technologies for the production of pipes made of cementitious material however are not without processing drawbacks.
One of the main problems which arise in production techniques by extrusion of cement-based pipes, is represented by maintaining the circular form at the outlet of the die.
Pipes produced by extrusion have the problem at the outlet of the die of maintaining their form as, due to their weight and low thickness, they bend over themselves losing their circular shape.
The lower the thickness of the extruded profile and with high vacuum percentages of the end-product the more significant this technical problem becomes.
The “vacuum percentage” refers to the percentage ratio between the empty surface and the full surface of the tubular product. The greater this percentage, especially in the presence of large dimensional end-products, the more critical the problem of maintaining the form becomes.
This problem is not limited to the field of cement-based pipes but also relates to pipes made of plastic materials such as for example PVC and PE pipes. In the field of plastic materials, the problem has at least been partially overcome by passing the pipe into a cooled calibrator which, by causing the rapid hardening of the plastic, also ensures its circular form.
This technical solution however can only be applied to plastic materials because, as these are extruded at high temperatures, their cooling causes hardening consolidating their shape.
On the contrary, the technical problem of maintaining the circular shape remains unsolved in the field of end-products and cement-based pipes as, contrary to what occurs for plastic materials, the extrusion is carried out under thermo-controlled conditions.
The problem of preserving the form for fibre-cement pipes having a low thickness is further increased by the high market demand for this type of fine piping. A greater vacuum percentage of the pipe section does in fact correspond, with the same nominal diameter, to a greater lightness of the pipe and consequently a lower cost per linear meter of the end-product.
Under normal extrusion process conditions, however, the fine thickness of the pipe can cause a loss in its circularity which, on the other hand, must be guaranteed in the hardened product to allow its final acceptability.
In the field of the invention, this characteristic is also defined as the “green strength” of the extruded product, or also “form stability”.
“Green strength” or “form stability”, in the present invention relate to the capacity of the neo-extruded end-product of maintaining its own shape (or geometry) immediately after leaving the extruder die.
This concept is widely described in U.S. Pat. No. 5,658,624, mentioned above with reference to the extrusion of pipes.
The possibility of obtaining an adequate green strength of the extruded product is typically related to various composition or process parameters, such as: compactness of the solid components; the low water/solid ratio of the paste also correlated with the mechanical resistance of the material; the extrusion pressure; the possibility of using a heated die; the possibility of using chemical compounds capable of being thermally activated to harden the outgoing material.
It should also be noted that the problem relating to the difficulty of preserving the form does not even allow tubular end-products having an adequate length to be obtained.
A further development of the above patent is represented by the process described in U.S. Pat. No. 5,545,297 in which a complicated mechanical system for continuous filament winding is introduced downstream of the die, for obtaining pipes with a high resistance and low thicknesses. The winding system also allows more rigid pipes to be obtained which preserve their circular form. The system described however is somewhat complex and expensive and does not adequately solve the problem.
Another document which refers to maintaining the circular form of extruded pipes consists of international patent application WO 2005/050079 A1 in the name of Rocla Pty Ltd. This describes the production of fibre-cement pipes having a low thickness, by means of a particular dewatering extrusion process which comprises eliminating the water from the material during extrusion. The level of the final water/binder ratio is in the order of 0.20, congruent with what is indicated in literature, for obtaining adequate mechanical resistance and therefore, in this case, high performance pipes with a low thickness.
Not even in this case however is the problem of maintaining the circularity after extrusion satisfactorily solved, as in the description it is stated that a substantially constant section of pipe length, not necessarily circular, is accepted.
In the current state of the art, the technical problem of the bending of fibre-cement pipes at the outlet of the extrusion die, which occurs as a result of their weight and fine thickness, has consequently remained unsolved.